Ensure that there's always a poller for completion

The C core uses completion_queue_next as a directive that progress should be made on requests bound to that cq.
If nobody is polling for completion, then a deadlock can occur (client needs to do work, but we're blocked waiting for the server).
There is no scope for this to occur using the idiomatic layers (as far as I can tell), but these low level tests need to be massaged.